Method for continuous cooking of chips in the manufacture of pulp



April 16, 1957 D. N. oBENsHAlN METHOD Foa CONTINUOUS cooxmc UF cPuPs 1N THE MANUFACTURE oP PULP 2 Sheets-Sheet 1 Filed Dec. 6, 1951 April 16, 1957 D. N. oBENsHAlN METHOD FOR coNTrNuous cooxmc: oF CHIPS IN THE MANUFACTURE 0F PULP 2 Sheets-Sheet 2 Filed Dec. 6, 1951 @Stb Il* WN United States Patent O METHOD FOR CONTINUOUS COOKING OF CHIPS IN THE MANUFACTURE OF PULP David Noel Obenshain, Piedmont, W. Va., assignor to West Virginia Pulp and Paper Company, New York, N. Y., a corporation of Delaware Application December 6, 1951, Serial No. 260,268

8 Claims. (Cl. 92-11) My present invention relates to the art of continuously cooking chips of wood or other fiber-yielding materials in the preparation of pulp.

At the present time most chemical wood pulp is produced by the batch method; that is, the chips and cooking chemicals in aqueous solution are charged into a pressure vessel, heat is supplied and after an appropriate interval of time the contents are discharged as pulp and spent liquor. In the so-called semi-chemical processes, the softened chips produced by the batch method are reduced to pulp by mechanical means after discharge from the vessel.

It has long been recognized that a successful continuous pulping process has numerous advantages over a corresponding batch process. Thus in a continuous process raw materials, chemicals and steam for heating flow continuously into the system and the resulting pulp leaves it continuously. This permits smaller storage reservoirs, eliminates peaks in steam demand, and insures better recovery of heat from the continuously discharging pulp. Other advantages characteristic of continuous pulping are the elimination of time required for charging and blowing digesters and the production of more uniform, better quality pulps.

To this end considerable effort has been applied as revealed by the large number of patents granted on continuous pulping processes and apparatus. Very few, however, have attained commercial success and these only to a limited degree.

Some of these prior methods have failed because of mechanical ditliculties, but I find the principal cause of failure is that due to the introduction of strong cooking liquor along with the chips into a digester which is at cooking temperature. The relatively dry chips are immediately brought to high temperature surrounded by strong cooking liquor. This results in very drastic action of the liquor on the exterior parts of the chips, severely damaging the outside bers, while the interior portions receive inadequate treatment by the relatively spent liquor which eventually penetrates inside. (This does not occur quite so radically even in batch cooking since the batch starts out relatively cold and requires considerable time to reach cooking temperature, thus permitting the strong liquor to substantially penetrate the interior of the chips before the cooking temperature is reached.)

For the purpose of overcoming the above-mentioned difficulties, it has been frequently proposed to employ multiple stage systems using two or more cooking vessels; also of employing a single vessel having means for maintaining several separate temperature zones. In general these systems have proven unsuccessful due largely to their excessively complicated construction and operation.

I have now discovered an improved method of continuously pulping which differs essentially from the prior art in that I introduce, along with the chips, liquor only slightly stronger in active cooking agent than that discharged with the cooked chips, thereby avoiding damage thereto, but I cause this liquor to circulate co-currently through the advancing mass of chips at least twice as fast as the chip movement, and preferably in the order of l0 to 100 times as fast. Thus the dry chips, while brought quickly to temperature, are never at any instant contacted by strong cooking liquor. The following data will illustrate the point:

Approx. Liquor Circu- Volumetric Ratio oi Liquor Circulating Circulating lation in Flow to Chip Flow in Liquor Euter- Liquor Leav- 50/Tonper Digester ing Dtgester, ing Digester, Day Cong./l. Nalo g.l. NazO tinuous Digs-ster, G. P. M.

The above data are based on sulfate pulping of hardwood chips containing 45% moisture, using 17.5% NazO on the o. d. wood, cooked 5 hours at 160 C. The 1:1 ratio above is typical of much of thc prior art. I have found that at substantially above 20 g./l. NazO the hot liquor tends to degrade the pulp in both quality and yield. Therefore, I prefer to operate in the ratio range of 10:1 to :1. Over the 100:1 ratio would require excessive circulating pump capacity and power with no appreciable gain, as can be seen from the above figures. It should be noted that in the foregoing table the exiting liquor has a concentration in all examples of 8.0 g./l. NazO. This value is, of course, a function of the parameters of the system. It must obviously be set to provide, in accordance with standard pulping technique, an exiting liquor concentration high enough to prevent reprecipitation on the fibers of the dissolved materials, which would degrade the pulp and make it difficult to bleach.

A surprising fact about the process is that the low concentration cooking liquors employed cook the chips quite adequately with approximately the same time and temperature schedule as employed in batch cooking. Furthermore, chips cooked to the same degree as comparable batch cooks give higher pulp yields and viscosities.

My improved process also lends itself advantageously to the use of a plurality of cooking vessels in which treatment by one solution in a tirst vessel is followed by a different treatment in a second vessel; for example, in the manufacture of alpha pulp, the chips may be hydrolyzed in the first vessel and then given an alkaline or other type of cook in the second vessel. Other modifications and adaptations of my process will occur to those skilled in the art.

My invention may be best understood by reference to the detailed description which follows, taken with the annexed drawings in which:

Figure 1 is a flow diagram illustrating the basic process as used in making fully cooked chemical pulps;

Fig. 2 is a ow diagram illustrating the process as applied to the manufacture of semi-chemical pulps; and

Fig. 3 is a llow diagram illustrating the process for two or more stage cooking such as used in the production of high-alpha pulps.

Referring to Fig. l, chips are supplied to chip feeder 1 which continuously feeds them (without crushing) into the digester 2 together with a stream of circulating cooking liquor from pipe 3. The cooking liquor flow rate is maintained so as to produce a relative rate of ilow through the mass of chips much greater than the rate of chip tlow, for example, a 40:1 ratio by volume and at an entering concentration of, for example, 11.1 grams per liter active cooking agent. The entering chips fall down on top of the mass of chips already in process together with the liquor which passes on through. In the course of time, for example, hours, the chips now fully cooked reach the bottom of the digester and are discharged through a continuous discharge mechanism 4 to a blow chamber 5 and thence on as pulp to the usual washers, screens, etc. Part of the cooking liquor is, ol course, discharged with the cooked chips. The major part of the cooking liquor, now considerably depleted, for example, to 8 grams per liter, is separated from the chips near the bottom of the digester by means of strainer bottom 6, flows through pipe 7, circulating liquor pump 8, heater 9 and pipe 3, back to chip feeder 1, and thence back into the digester. The concentration of active cooking agent in the circulating liquor is raised back to its original value of 11.1 grams per liter by means of fresh strong cooking liquor from supply tank 10, pump 11 and pipe 12. Preferably the discharge mechanism 4 removes only enough liquor with the chips to keep the liquor level in the digester at a predetermined height which is somewhat below the chip level to avoid floating of the chips.

Fig. 2 illustrates how my process may be carried out in the production of semi-chemical pulps. In this process the chips are not cooked to the point where they will deber upon blowing, but are discharged as softened chips which have to be debered by additional mechanical means. The process is substantially the same as in Fig. l until the semi-cooked chips reach the discharge mechanism 4a. Here a portion of the liquor is separated from the chips and is returned to the digester circulating system by way of pipe 13a. The partially drained chips are ilushed from discharge mechanism 4a and transported to chip drainer 14 by means of circulating waste liquor from tank 1S and pump 16. The drained chips from 14 go on to a detibrator 17 and leaving here as pulp proceed on to the conventional washers, screens, etc. not shown. The drained liquor from 14 returns to the waste liquor tank for recirculation.

Pig. 3 shows how my process may be operated for multi-stage cooking and illustrates the process as applied to the manufacture of high-alpha pulps in which the rst stage is a hydrolysis with acid or water and the second stage is a sulfate cook. The hydrolysis proceeds in digester 2b in which the chips and liquor are moved exactly the same as the semi-chemical process explained in connection with Fig. 2. As shown, the hydrolyzed chips are ushed from the discharge mechanism 4b by the circulating liquor from the sulfate cooking digester 2c and transported by such liquor to the top of digester 2c. Cooking proceeds here exactly as in the arrangement of Fig. l, the cooked chips being discharged as pulp from discharge mechanism 4c into blow chamber 5c.

Still another modification of my improved process not illustrated in the drawings is as follows: Two digesters are provided as in producing high-alpha pulps. The rst digester uses the spent liquor from the second digester as its cooking liquor. This spent liquor while low in active cooking agent still has appreciable cooking agent present. This active agent is readily consumed in the tirst digester by the raw chips, leaving practically no active cooking agent in the exit liquor which goes to the liquor recovery processes. Thus appreciable economy may be realized in the amount of active cooking agent required.

Obviously any number of stages could be used in producing pulps of special characteristics or for conservation of chemicals or for other reasons.

My improved method may thus be used for producing pulps of various kinds, such as full chemical pulps, semichemical pulps, high alpha pulps, etc.

The following typical cooking data will serve as a guide in the carrying out of the processes for the production of the various types of pulps described above. Data for a batch sulfate cook for producing as nearly as possible the same quality of pulp from the same kind of wood are also included for comparison:

Continuous sulfate pulpng Wood Oak. Cooking agent NaOH-l-NazS. Sultidity 25%. Circ. liquor entering 10 g./l. active NazO. Circ. liquor leaving 7 g./l. active NazO. Liquor to chip ow ratio 40:1. Total cooking time 5 hours. Temperature 160 C. Yield 53%. K No. 13.7. Viscosity 4200. Lb. steam per lb. pulp 1.2.

Batch sulfate pulping Wood Oak. Cooking agent NaOH-NagS. Sultidity 25%. Charging liquor 54 g./l. active NazO. Black liquor at finish 5 g./l. active N320. Total cooking time 5 hours. Temperature 160 C. Yield 48%. K No. 13.7. Viscosity S27. Lb. steam per lb. pulp 2.0.

Continuous semi-chemical pulpr'ng Wood Mixed hardwoods. Cooking agent Na2SO3+Na2CO3. Circ. liquor entering 38 g./l. Circ. liquor leaving 33 g./l. Liquor to chip tlow ratio 40:1. Total cooking time 6.5 hours. Temperature 175 C. Yield 70%. l b. steam per lb. pulp 1.5.

Continuous high-alpha pulping Wood Oak. Hydrolysis stage:

Hydrolysis agent HNOs. pH entering 1.32. pH leaving 1.48. Liquor to chip ilow ratio 40: 1. Hydrolysis time 3.5 hours. Temperature C. Cooking stage:

Cooking agent NaOH-l-NafzS. Sultidity 25%. Concentration entering 13 g./l. NazO. Concentration leaving 10 g./l. NazO. Liquor to chip ow ratio 40: 1. Cookingtime Shours. Temperature C. Total steam lb. per lb. pulp 2.3.

The apparatus for carrying out my improved process may be of presently known types; or preferably it may consist of improved apparatus as described and claimed in my copending below identified patent applications which are hereby incorporated herein by reference:

Serial No. 260,269, led December 6, 1951, entitled Charging and Discharging Mechanism for Use in Con tinuous Cooking of Chips in the Manufacture of Pulp, now Patent No. 2,680,683;

Serial No. 260,270, filed December 6, 1951, entitled Strainer Mechanism for Use in Continuous Cooking of Chips in the Manufacture of Pulp, now Patent No. 2,680,684;

Serial No. 260,271, led December 6, 1951, entitled Chip Level Controller for Use in Continuous Cooking of Chips in the Manufacture of Pulp," now Patent No. 2,680,298.

I claim:

1. In a process for manufacturing pulp from wood or other tibrous materials including chemical cooking of such material, the steps of continuously introducing said material to a digester and feeding said material through said digester from an inlet to an outlet, continuously circulating a chemical cooking liquor through said digester cocurrently with said material, said material being untreated by said liquor prior to introduction into said digester, the liquor being introduced at said inlet and all flowing cocurrently with said material to said outlet, separating the liquor from the cooked inaterial at said outlet and recirculating the liquor to said inlet, fortifying the recirculated liquor with fresh liquor, heating said recirculated liquor to the maximum digestion temperature employed for said liquor throughout the process, and reintroducing the thus heated liquor into the digester at said inlet concurrently with a fresh supply of said untreated material to intermingle the heated liquor with said untreated material, said reintroduced liquor having a concentration below that which would degrade the pulp at said maximum cooking temperature, the rate of circulation of liquor to the rate of feed of brous material through the digester being in the ratio of at least approximately 10:1 by volume, said cir culation rate and said concentration of reintroduced liquor being related to provide a liquor concentration at the outlet suiiicient to prevent reprecipitation of the dissolved matter.

2. In a process as defined in claim 1, said cooking liquor being of the sulfate type.

3. In a process as dened in claim 2, said sulfate cooking liquor having a concentration of approximately 20 grams or less per liter NazO as it is introduced to the digester at said inlet.

4. In a process as defined in claim l, said cooking liquor being of the neutral sodium sulte type.

5. In a process as defined in claim 1, said cooking liquor being of the acid hydrolyzing type.

6. In a process as defined in claim 5, the fibrous material obtained at the outlet of the digester being transferred to a second digester and being there subjected to an alkaline cooking liquor digestion.

7. In a process as delined in claim l, said ratio being from approximately 10:1 to approximately 100:1.

8. In a process as defined in claim 1, said ratio providing a concentration of the liquor cooking values at the outlet of at least approximately 40% of the cooking values of the introduced liquor at the inlet.

References Cited in the file of this patent UNITED STATES PATENTS 1,505,934 Olier Aug. 19, 1924 1,690,954 Spencer Nov. 6, 1928 1,915,812 Wollenberg June 27, 1933 1,933,609 Wagner Nov. 7, 1933 1,938,802 Braun Dec. 12, 1933 1,970,148 Pomilio Aug. 14, 1934 1,991,244 De la Roza Feb. 12, 1935 2,008,635 Brubacher July 16, 1935 2,018,937 Wells et al. Oct. 29, 1935 2,089,992 Campbell et al. Aug. 17, 1937 2,200,034 Merrill May 7, 1940 2,287,332 Steely June 23, 1942 2,466,290 Wells Apr. 5, 1949 2,542,801 De la Roza Feb. 20, 1951 

1. IN A PROCESS FOR MANUFACTURING PULP FROM WOOD OR OTHER FIBROUS MATERIALS INCLUDING CHEMICAL COOKING OF SUCH MATERIAL, THE STEPS OF CONTINUOUSLY INTRODUCING SAID MATERIAL TO A DIGESTER AND FEEDING SAID MATERIAL THROUGH SAID DIGESTER FROM AN INLET TO AN OUTLET, CONTINUOUSLY CIRCULATING A CHEMICAL COOKING LIQUOR THROUGH SAID DIGESTER COCURRENTLY WITH SAID MATERIAL, SAID MATERIAL BEING UNTREATED BY SAID LIQUOR PRIOR TO INTRODUCING TION INTO SAID DIGESTER, THE LIQUID BEING INTRODUCED AT SAID INLET AND ALL FLOWING COCURRENTLY WITH SAID MATERIAL TO SAID OUTLET, SEPARATING THE LIQUOR FROM THE COOKED MATERIAL AT SAID OUTLET AND RECIRCULATING THE LIQUOR TO SAID INLET, FORTIFYING THE RECIRCULATED LIQUOR WITH FRESH LIQUOR, HEATING SAID RECIRCULATED LIQUOR TO THE MAXIMUM DIGESTION TEMPERATURE EMPLOYED FOR SAID LIQUOR THROUGH OUT THE PROCESS, AND REINTRODUCING THE THUS HEATED LIQUOR INTO THE DIGESTER AT SAID INLET CONCURRENTLY WITH A FRESH SUPPLY OF SAID UNTREATED MATERIAL TO INTERMINGLE THE HEATED LIQUOR WITH SAID UNTREATED MATERIAL, SAID REINTRODUCED LIQUOR HAVING A CONCENTRATION BELOW THAT WHICH 